1. Field of the Invention
This invention relates to an improved process for photowriting optical gratings and arrays of gratings into glass fibers, and the gratings and arrays of gratings produced by this process.
2. Description of the Related Art
Optical gratings in glass fibers are finding their way into an ever-growing number of applications, including communications systems, strain sensing systems, and optical processing and computing systems. Optical gratings in glasses are most typically laser-induced, i.e. photowritten into glasses by interfering laser beams. See, e.g., U.S. Pat. No. 4,807,950, "OPTICAL FIBER IMPRESSED REFLECTION GRATINGS", issued Feb. 28, 1989 to Meltz et al. These gratings selectively reflect axially transmitted light.
Production of these gratings in glass fibers by available methods is highly unsatisfactory. Consequently, when these gratings are available at all, they cost several thousand dollars apiece. More commonly, however, manufacturers who want to incorporate these gratings into their systems will find that the gratings are simply unavailable at any price.
A typical manufacturing scheme involves the following steps: The jacketing is carefully removed from a segment of glass fiber, under clean room conditions, with great care being taken to minimize contact with the unjacketed fiber surface. The fiber is then positioned in a laser interferometer, with steps taken to isolate the fiber from external vibrations, air currents, and other sources of unwanted motion. The fiber is then transversely exposed to between several hundred to several thousand shots from interfering pulsed light beams, which are usually formed by splitting the beam from a writing laser. This usually takes anywhere from several tens of seconds to several minutes exposure. Alternatively, the writing laser may be a cw laser. After the fiber is sufficiently exposed to form a grating with a useable index of refraction modulation, .increment.n, the grating is carefully rejacketed, again with great pains being taken to avoid unwanted handling of the fiber.
There are several adverse consequences to this process. One consequence is that these hundreds or thousands of laser shots must be superimposed on top of one another with great precision, or no grating will result. Problems that prevent the precise superpositioning of multiple laser shots include vibrations, air currents within the interferometer arms, thermally induced dimensional changes, and pointing instabilities in the laser.
Moreover, this process is not precisely controllable. Consequently, the grating-writing process must be continuously monitored as it is carried out. This is typically done by probing the grating with light guided in the fiber core as the grating is written, and monitoring the reflected signal from the probe beam.
Another problem with this method is that it requires complex processing steps and a great deal of handling of the fiber. Newly drawn and coated glass fibers have excellent tensile strength, provided the quality of the glass surface is good. This strength is tremendously sensitive to the quality of the glass surface. Because grating writing is most effectively done on a bare (unjacketed) fiber, careful stripping, cleaning, and immobilization of the fiber prior to the lengthy exposure step are necessary to preserve as much of this strength as possible. After exposure, the bare fiber must be rejacketed to maintain its remaining surface quality (and consequent strength). Inevitably, there will be some damage to the glass surface, with some consequent loss of fiber strength.
An additional disadvantage of this process is that it is necessarily a batch process, carried out on a single fiber segment at a time, after that segment is drawn, coated, cut, and stripped of its jacketing. This has the disadvantage of making high speed production runs unavailable. Another disadvantage to batch processing is that large arrays of gratings (such as the arrays that would be used for distributed strain sensing) are assembled by splicing together individual grating segments. These splices are typically not as strong as the fibers they connect. This makes these assembled grating arrays subject to failure under strain. Furthermore, these splices will cause some transmission loss, and may contribute to spurious reflections.
The problems of having gratings with low tensile strength connected by splices with low tensile strength is particularly critical because one of the most important envisioned uses for these gratings is in strain sensing systems. See, e.g.. U.S. Pat. No. 4,806,012, "DISTRIBUTED, SPATIALLY RESOLVING OPTICAL FIBER STRAIN GAUGE", issued Feb. 21, 1989, to Meltz et al. Strain sensor arrays that fail under strain are of clearly limited usefulness.
There are also disadvantages inherent in the gratings produced by this method. For example, gratings produced by this method are not thermally stable. These gratings will disappear after only a few minutes at fairly moderate temperatures (for instance, at temperatures above 300.degree. C. these gratings disappear rapidly). This makes these gratings unsuitable for use in systems where they would be exposed to high temperatures. These high temperatures may be encountered as part of the manufacturing process, as long-term exposure during normal system operation, or as occasional transient exposure during system operation. For example, strain sensing arrays in composite airframes are highly desirable. However, incorporating composite materials into high temperature applications would require strain sensing gratings of proven thermal stability.